This invention relates to an optical wavelength filter that is operable at ultra-high speeds in the nanosecond range and that is tunable over a wide range.
Optical wavelength filters are useful for the advanced fiber optic links used, for example, in optical Wavelength Division Multiplexing (WDM) networks. It is well known that there is an exponentially growing demand on the data transmission bandwidth for both civilian and military applications. Fiber optic links and networks have become the backbones for data transmission with large bandwidth.
In terms of military applications, fiber optic link technology has the unique bandwidth capability, the immunity from electromagnetic interference (EMI) and crosstalk, the light weight and the electrical performance necessary to realize fast data rates and reduced signature multi-function antenna apertures. However, a major remaining issue associated with military fiber-optic systems is high cost. In commercial fiber optic systems, the transmission bandwidth has been enhanced without substantially increasing cost by using WDM networks. WDM technology also has great potential to reduce the cost for military fiber-optic systems by cutting down the number of fiber optic lines and connectors.
In addition, since analog and digital data transmission is dominant in military fiber-optic systems, it is very important to have packet-level and cell-level switching capability in the WDM system so that efficient data transmission can be achieved. To realize packet-level and cell-level switching capability, there is a need for optical filters that are capable of ultra-high-speed (nanosecond) manipulations and of very fast tuning speed.
On the other hand, it is also a very challenging task to develop such a ultra-high speed dynamic WDM network due to the fact that network functionality requires dynamic elements to perform signal processing manipulations at different levels of complexity for circuit as well as packet-level and cell-level switching networks. This functionality includes filtering, routing, add-drop multiplexing, wavelength conversion, optical cross-connects, header reading, and so on. Among these functions a key element is a tunable optical filter.
Since current commercially available dynamic elements, such as Fabry Perot (FP) tunable filters are relatively slow, current dynamic WDM technology relies on relatively low dynamics (i.e., up to millisecond speeds), which is most adequate for circuit switched applications. However, network functionalities such as packet-level or cell-level switching needs much faster speed (i.e., in the nanosecond range). Due to the lack of commercially available ultra-high speed dynamic elements such as tunable filters, currently, packet-level or cell-level switching still has to be implemented by electronics, which limits the huge bandwidth benefit of light signals, increases cost and weight, and reduces the robustness against EMI. In other words, the lack of optical packet-level and cell-level switching becomes a bottleneck for advanced fiber optic links in high speed dynamic WDM networks.
To meet the needs of WDM fiber optic networks, in recent years, a variety of tunable optical filters have been developed. These include Fabry Perot (FP) interferometer tunable filters, Ferroelectric liquid crystal FP filters, micro machined device filters, Mach-Zehnder interferometer (MZI) filters, Fiber Bragg grating (FBG) filters, acousto-optic tunable filters (AOTF), electro-optical tunable filters (EOTF), arrayed waveguide grating (AWG) filters, optical MEMs, active filters. Filter performance is evaluated by filter parameters that include insertion loss, bandwidth, sidelobe suppression, dynamic range, tuning speed, and cost.
Referring to FIG. 1, Table 1 summarizes the performance of the above types of filters. Among these filters, FBG and Fiber FP filters are most commercialized, mainly due to the fact that no medium transformation is required so that the filters are low cost, robust and easy to use. However, FBG and Fiber FP filters are inherently limited in tuning speed to the millisecond range due to their thermal or mechanical mechanisms. Thus, FBG and Fiber FP filters can not be used for packet-level or cell-level switching, in which nanosecond tuning speed is required. On the other hand, although tunable filters based on faster mechanisms such as electro-optic effect can have nanosecond tuning speed, they are still on the research stage. A major impediment to commercialization may be due to complexity and cost. Since this category of filters is not fiber based, medium transformation is required when connected in a fiber optic WDM network. This increases the complexity of the usage and cost.
ETOF filters have wide bandwidth and strong sidelobes. To reduce the bandwidth and strong sidelobes of EOTF, tunable narrow-band filters have been constructed with photorefractive LiNbO3 fibers and bulk crystals. Bragg gratings are holographically written inside the LiNbO3 materials so that narrow bandwidth with low sidelobe can be achieved. Since photorefractive LiNbO3 materials are also electro-optic materials, the refractive index of the material can be fast tuned by the external electric field. The tuning speed can be in the nanosecond range, which is fast enough for the packet-level and cell-level switching. When the refractive index is changed, the effective Bragg grating period is also changed so that the wavelength response of the filter can be tuned. Although a very narrow bandwidth low sidelobe fast tuning speed filter can be synthesized, the tuning range of the filter is very limited. The wavelength tuning range of this Bragg grating filter can be estimated as                               Δλ          ≈                                                    Δ                ⁢                                  xe2x80x83                                ⁢                n                            n                        ⁢                         ⁢            λ                          ,                            (        1        )            
where n is the refractive index of the material, xcex94n is the refractive index change, and xcex is the operating wavelength. Substituting typical values for LiNbO3 materials, (i.e., n=2.3, xcex94n=10xe2x88x923, and xcex=1500 nm) into Equation (1), one can obtain xcex94xcex less than 1 nm. Obviously, this tuning range is too small for practical use in a dense WDM network. In addition, medium transformation is also required in this type of filter, which further increases the difficulty in commercialization.
Long period gratings (LPG""s) that are photoinduced fiber devices couple light from the core of a single-mode optical fiber into a fiber cladding at discrete wavelengths, producing one or more attenuation bands in the fiber transmission. The phase-matching condition of a LPG can be written as
xcexp=(ncoreeffxe2x88x92ncladeff)xcex9, xe2x80x83xe2x80x83(2) 
where xcexp is the wavelength of the pth-order resonance peak, xcex9 is the period of the grating, and ncoreeff and ncoreeff are effective indices of core and cladding, respectively. Based on Equation (2), the wavelength tuning range xcex94xcex for the long period grating can be estimated as                               Δλ          =                                                    Δ                ⁡                                  (                                                            n                      core                      eff                                        -                                          n                      clad                      eff                                                        )                                                                              n                  core                  eff                                -                                  n                  clad                  eff                                                      ⁢                          λ              p                                      ,                            (        3        )            
where xcex94(ncoreeffxe2x88x92ncoreeff) is the difference of the effective refractive index change between the core and cladding. Since the effective refractive indices of core and cladding can be very close, i.e., ncoreeffxe2x88x92ncladeff less than  less than 1, a small change in the ambient refractive index can result in a big wavelength shift. Thus, a wide tuning range can be achieved. A 50 nm tuning range filter is described by A. Abramov, A. Hale, R. Windeler and T. Strausser in an article entitled Widely Tunable Long-period Gratings in Electrical Letters, vol. 35, pages 81, 82, 1999. Although wide tuning range was achieved, the tuning speed is very limited due to the use of low speed thermal tuning.
The optical filter of the present invention is tunable to different wavelengths in a portion of the spectrum, such as the infrared portion. The optical filter includes a core with a long period grating disposed thereon. A first cladding layer is disposed on the core. A second electro-optic cladding layer is disposed on the first cladding layer. The first cladding layer has an ultra-thin thickness that supports a single resonant band over the portion of the spectrum. The resonant band is tunable to the different wavelengths by a voltage applied to the second electro-optic cladding layer.
The core and the first cladding layer are formed of silica fiber material. The long period grating is fabricated in the fiber core. The refractive index of the electro-optic layer can be tuned by applying the voltage to transparent electrodes disposed thereon. By controlling the voltage across the electro-optic cladding layer, the wavelength spectrum of the filter can be fast and widely tuned.
According to one aspect of the invention, the electro-optic layer is a polymer that has a refractive index lower than the refractive index of said silica fiber material. In some embodiments, the polymer is a copolymer. Preferably, the copolymer is poly(vinylidene flouride-trifluoroethylene).
The optical filter of the present invention has fast tuning speed (nanosecond range), wide tuning range ( greater than 50 nm), low insertion loss ( less than 0.1 dB), narrow bandwidth ( less than 0.5 nm), and low sidelobe ( less than 30 dB).
In addition, by taking advantage of wavelength division multiplexing, the total number of fiber optic links required in an optical network is also reduced, which in turn reduces the cost of the fiber optic links. The filter of the invention can be used for high speed packet-level and cell-level switching, which is critical for the high-bit-rate data transmission.
In a broader aspect, the present invention is an electro-optical device that includes a core with a long period grating disposed thereon. A first cladding is layer disposed on the core. A second electro-optic polymeric cladding layer is disposed on the first cladding layer.